1. Field of the Invention
The present invention relates to the microencapsulation of a variety of materials by interfacial polyaddition, and, more especially, to the preparation of microcapsules whose wall members, or outer skins, comprise a polyurea.
2. Description of the Prior Art
U.S. Pat. No. 3,577,515 describes the encapsulation of various materials by interfacial condensation of additional organic intermediates capable of forming polyamides, polysulfonamides, polyesters, polycarbonates, polyurethanes or polyureas. These additional organic intermediates are present in substantially immiscible separate liquids, such as, in particular, water and a water-immiscible organic solvent. Thus, the '515 patent exemplifies the preparation of capsules having polyurea skin membranes by interfacial condensation of diisocyanates or polyisocyanates with a diamine and/or a triamine.
The use of many diisocyanates or polyisocyanates has been described in later patents. Thus, 1,6-diisocyanatohexane is mentioned in particular. The use of 1,6-diisocyanatohexane trimers has also been recommended, the trimer either containing a biuret group, or an isocyanurate ring. For example, published European patent application No. 40,770 describes a process for the preparation of aqueous suspensions of microcapsules by interfacial polyaddition of an aliphatic polyisocyanate containing an isocyanurate group with a compound containing active hydrogen, the pH being adjusted, upon completion of the polyaddition, to a value equal to or lower than 7.
Published French patent application No. 2,498,474 describes a process for the preparation of microcapsules by emulsifying, in a hydrophilic liquid, a hydrophobic liquid containing an aromatic polyisocyanate and an aliphatic isocyanate, such as the 1,6-diisocyanatohexane trimer containing a biuret group or the 1,6-diisocyanatohexane trimer containing an isocyanurate ring. The use of an aromatic polyisocyanate, however, presents the disadvantage of frequently producing discolorations which are undesirable, especially for papermaking applications.